1. Field of the Invention
The present invention relates to a method synthesizing Cu(In,Ga)S2 nanoparticles/wires based on metal salts.
2. Description of the Prior Art
A CIGS thin film is prepared by the formation of semiconductor nanoparticle and nanowire networks and selenization for a light absorption layer of photovoltaic devices.
Chalcopyrite CuInGaSe2 CIGSe is a direct band gap semiconductor and has an exceptionally high absorption coefficient of more than 105/cm for 1.5 eV and higher energy photons. According to the recent report from the ZSW, a solar cell based on CIGSe has reached a power conversion efficiency of 20.3%, which is comparable with the energy conversion efficiency of crystalline Si solar cells. Decent conversion efficiency and high chemical stability of CIGSe make itself a promising p-type material for thin film photovoltaic devices.
Vacuum and non-vacuum technologies are the two main methods of preparing CIGSe thin films. Vacuum-based processes including co-evaporation and sputtering, which have been successfully applied in commercial production lines. However, the high cost and complexity of vacuum-based fabrication process become barriers to affordable commercial modules.
An efficient non-vacuum printing process has the potential to overcome this barrier. The low cost technique is inherently suitable for large-scale applications and may benefit from established industries of coatings, paints, inks, electronic ceramics and colloidal systems. In particular, deposition at atmospheric environment offers an opportunity for the deposition of absorber materials at large scale with high throughput. This provides a potential cost advantage over conventional fabrication process that involves expensive vacuum equipment.
Kapur et at (U.S. Pat. No. 6,127,202) describe a method for fabricating a CIGSe solar cell based upon the solution-based deposition of a source material comprised of mechanically milled, oxide-containing, sub-micron sized particles, while Eberspacher and Pauls (U.S. Pat. No. 6,268,014); Published U.S. Patent Application No. 2002/0006470) describe the forming of mixed metal oxide, sub-micron sized particles by pyrolizing droplets of a solution, then ultrasonically spraying the resulting particles onto a substrate. However, the high-temperature hydrogen reduction step is potentially explosive and requires substantial time and energy. Meanwhile, highly toxic H2Se gas atmosphere is requested in the selenization process. Byoung Koun Min in Published U.S. Patent Application No. 2012/0080091 A1 also involves the reduction process.
Fuqiang Huang in Published U.S. Patent Application No. 2011/0008927 A1 gets a 14.6% high efficiency employing a non-vacuum liquid-phase chemical technique.
David B. Mitzi. in Published U.S. Patent Application No. 2009/0145482 also gets above 10% efficiency CIGSe thin film solar cells using hydrazine as the solvent source.
Nanosolar Inc. in Published U.S. Patent Application No. 2008/0149176 has used binary copper selenide and indium/gallium selenides nanoparticles as starting materials to fabricate 14% thin film CIGSe solar cells. Single metallic nanoparticles are the simplest form one could design. The structure of nanoparticles used by Nanosolar has a core-shell structure. Copper nanoparticles serve as the cores which are coated with IIIA-VIA shells such as indium selenide, gallium selenide etc. These selenide nanoparticles are dispersed in organic solution which may contain various ingredients including solvents, surfactants, binders, emulsifiers, thickening agents, film conditioners, anti-oxidants, flow and leveling agents, plasticizers and preservatives. By using the similar core shell strategy, Yoon et al. synthesized CuSe/InSe nanoparticles which yield only ˜1% efficiency.
However, those methods mentioned here require toxic reagents, need inert gas protection, require complex processes and are not easy to scale up to mass production. Thus, there is a need in the art, for a non-oxide, nanoparticle based precursor material that overcomes the above disadvantages.